Between July 19th and August 31st, owners of the Japanese version of Pokemon Ranger will be able download a special Shaymin mission. Extra missions are only available by downloading them from Nintendo Wifi Connection on your Nintendo DS. In order to download extra missions, it is necessary that you have completed the game.

"The mission, For the Sake of the Bride and Shaymin!, takes place at a wedding and has the player find and protect Shaymin from Team Yamiyami. " (Bulbanews)

Pokemon Ranger Batonnage is an action-adventure game for the Nintendo DS and the sequel to Pokemon Ranger. It was released in Japan on March 20, 2008. The game is set in a new region called Almia where the base of operations is a Ranger School that the main characters have just graduated from.

Instructions for downloading the extra missions are available on the official Japanese Pokemon website. You can view the instructions by clicking here.

Unlike in the special missions, the Pokemon captured in the extra downloadable missions are subsequently released without allowing them to be transferred to Pokemon Diamond and Pearl.

Source: http://www.pokemon.co.jp/special/ranger/game_info/06.html